We want a high powered rifle with the following characteristics (note: this may be impractical and we need to talk my co-author down)
- Sniper's rifle suitable for hunting
- Adjustable scope with subvocal or gesture focusing (subvocal words through transceiver in jaw, or by squinting)
- Switchable ammunition. Can switch between penetration rounds, high impact rounds, others without having to change out magazine. Co-author would like bullets manufactured
- Not gunpowder based. So rail gun?
The AR-15 platform is your ideal for this. Can be built easily out of machine tools for any caliber, very simple to maintain, and the most common firearm in the US. They are less robust than the AK-47, but just as easy to make and use. They are *exceptionally* accurate, as well, and in the hands of an experienced operator are good out to about 500yds (sometimes 1000, if you have a very good shooter). There are a few other platforms that can serve as well, but none have the same versatility or rate of fire, and if you're outfitting a small town that has to be more-or-less self-sufficient, pretty much everyone's going to standardize on one platform. The AR platform is good for this because you can use it for everything from a pistol to a machine gun with very minor modifications, and all those mods can be done by hand with a screwdriver in the field.
The scope is a bolt-on accessory, so as long as you've got a way to get ahold of them (stealing from an abandoned military base?) you're golden at least as long as the batteries and electronics last. I've gotta wonder: Why the hell would you *want* subvocal or gesture control for a scope? As a longtime shooter, I can't imagine these things would do anything but get in the way.
Ammo switching is something you can do in any magazine-fed firearm - just stack the bullets in the order you'll want to use them. We probably oughta have a long discussion on the different types of rounds you're thinking of, and whether there's any actual difference between them. Bullet designs are not infinite - there's only about 8 general types. By shape:
And by performance characteristics:
Some are limited by physics in terms of how fast they can fly. The spitzer, for example, is the only practical load for round that goes faster than about 1600fps - other shapes simply slow down too fast to make it worth it. Shot capsules, similarly, tend to fracture prematurely if fired at high speed, and don't fly very far anyway. Frangibles are strictly a subsonic affair and are designed to shatter upon impact. Explosive rounds (at least ones that you can manufacture easily at home) are retro-fits on hollow points (i.e. drilling out and filling the cavity with unoxidized sodium or potassium or the like that explodes on contact with moisture, then sealing it with wax so it doesn't blow up in the air. Fun fact, sodium/potassium alloy is super reactive and makes a poor-man's tracer as well. The reduced mass makes these ineffective against hard targets but their moisture-reactivity make them utterly devastating to anything made out of meat).
There are other kinds of rounds (esp if you get into shotguns an military munitions), but those are the basic types.
Without gunpowder, you're pretty limited. Pre-compressed pneumatics can do pretty good out to about 100-200 yds, and you can carry extra pressure vessels with you. Capacity is pretty small, as you can imagine if you've ever gone paint-balling or used an air compressor, but you can get 4-8 shots at ballistic speeds out of each pressure vessel (unless you're carrying a SCUBA tank, in which case you can get quite a lot out of them).
Coilguns and rail guns are your other option, but you need a LOT of power and it has to dump *fast*, which means capacitors, electronics, and high-capacity high-draw batteries. That's potentially a lot of weight, as even with theoretical magic batteries you run into an energy density problem not too far away from where we're currently at (at least in a Moore's law sense). Might be worth doing some garage builds and testing (they're not hard to build) with some ballistics gel or a sand bucket to shoot into so you can work out the energy draw required for a given fps (which will let you determine the effective range through a rifled barrel by comparing them against performance tests for common firearms).
RE: bullet manufacture - he means manufacturing them locally? Not a problem. Lead is easy to cast. Lead supply and brass supply are your only major limiting factors (the latter is easily mitigated
though not solvedby policing your brass). Likewise, if you go for gunpowder, both nitrocellulose (the base of smokeless powder) and black powder are dead easy to make. Primers would be the limiting thing, but they can be manufactured from scratch as well if you have a sufficiently determined chemist/metalurgist. Mercury fulminate or nitroglycerine or other similar shock-detonating materials, a tin (or aluminum or copper) retainer cap, etc. A good pair of calipers and a reloading press, a little bit of expertise, and you've got ammo from scratch.